A rewritable optical disk such as a DVD-RAM has a phase change recording layer on its substrate. When this phase change recording layer is irradiated with a laser beam having a high energy density, the irradiated portion is locally heated to a temperature exceeding the melting point and melted. Since the optical disk being irradiated with the laser beam is spinning at a high velocity, the beam spot of the laser beam will be moving along the track on the phase change recording layer at a high velocity, too. That is why that portion of the phase change recording layer that has been melted by the passage of the beam spot is quickly cooled and solidified naturally. If the power of the laser beam is adjusted in such a situation, then the melted portion of the phase change recording layer is rapidly cooled and amorphized. The amorphized portion of the phase change recording layer has a different refractive index and a different optical reflectance from those of the other crystalline portions. The amorphized portion formed in this manner is called a “mark”. On the other hand, an intervening portion between those “marks” on the track is called a “space”.
By arranging those marks and spaces on the track, data can be recorded on the optical disk. If a laser beam with a low power for reading is radiated toward the optical disk and if the intensity of its reflected light is measured, then the mark/space boundary (which is often called a “mark edge”) can be sensed and data can be read. The power of the read laser beam is kept low enough to avoid melting the phase change recording layer.
To increase the information transfer rate while data is being read from, or written on, any of those optical disks, either the recording linear density or the scanning rate of the beam spot on the optical disk may be increased.
In order to increase the recording linear density, it is effective to reduce the mark length and space length or to narrow the mark edge position detecting interval by reducing the steps of variations in mark and space lengths.
However, if the recording linear density were increased, then the SNR of the read signal would decrease. For that reason, a significant increase in recording linear density should not be expected.
To make very small marks on an optical disk with high precision, a write strategy, in which each of those marks is left on the recording layer by continuously irradiating that layer with either a single laser pulse or multiple laser pulses, is adopted.
According to a conventional technique as disclosed in Japanese Patent Application Laid-Open Publication No. 5-298737 (which will be referred to herein as a “first conventional technique”), a train of laser pulses is assigned to each of multiple marks with different lengths. In other words, a train of laser pulses to be radiated to make each mark, i.e., a waveform showing the intensity variation of the laser beam (which will be referred to herein as a “write pulse waveform”), is determined by the length of that mark to leave. The number and amplitude of pulses to be radiated during the period of making each mark are controlled according to the length of a write code sequence.
The write pulse waveform during the mark making period can be divided into a top portion and a succeeding portion. The respective pulses generally have different pulse heights. Also, in the periods other than the mark making period, a write auxiliary pulse is generated to follow the space.
According to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 5-298737, the diffusion of heat from a preceding mark toward the front edge of the very next mark can be compensated for, and the mark width and mark edge position can be controlled with high precision, irrespective of the space length.
According to another conventional technique as disclosed in Japanese Patent Application Laid-Open Publication No. 8-7277 (which will be referred to herein as a “second conventional technique”), each write code is broken down into a plurality of primitive elements with multiple different lengths such that a single write pulse is associated with each of those elements. And each write code is formed by a series of recording marks associated with respectively independent write pulses.
Still another conventional technique as disclosed in Japanese Patent Application Laid-Open Publication No. 9-134525 (which will be referred to herein as a “third conventional technique”) adopts a multi-pulse writing method that uses the first heating pulse, a number of succeeding heating and cooling pulses that follow the first pulse, and the last cooling pulse. According to the third conventional technique, in recording a mark, of which the length is either an odd number of times or an even number of times as long as one period of a write channel clock, the pulse width of the succeeding heating and cooling pulses is made nearly equal to the length of one period of the write channel clock.
According to yet another conventional technique as disclosed in Japanese Patent Application Laid-Open Publication No. 11-175976 (which will be referred to herein as a “fourth conventional technique”), the energy and the number of pulses that are applied while a mark of an arbitrary length is being made are changed according to the length of the mark in a write code sequence such that the gap between two arbitrary variation points of the energy applied per unit time during the mark making period becomes longer than a half of the detection window width.
According to the first conventional technique, the length of a mark, corresponding with the detection window width, is associated with one shot of write pulse. Thus, if the detection window width is shortened, then the semiconductor laser diode, functioning as a source of generating write energy, needs to be driven faster than usual. For example, if one tries to realize a burst transfer rate of 10 megabytes per second, which is almost as high as that of a magnetic disk drive, by a normal (1, 7) modulation technique, then the detection window width of the read signal will be about 8.3 ns (nanoseconds) and therefore the shortest write current pulse width will be about 4.2 ns, which is approximately a half as long as the detection window width. However, it usually takes several nanoseconds to activate a semiconductor laser, and it is difficult to generate a write beam pulse accurately. Also, even if a write beam pulse could be generated accurately, normal marks could not be made in a situation where multi-pulse writing is carried out on a medium such as a phase change disk in which the mark making is controlled by the cooling rate of its heated portion. This is because the next beam pulse is radiated before the heated portion is cooled sufficiently. Also, if one tries to realize a burst transfer rate of 10 megabytes per second by the (1, 7) modulation technique, for example, then the amount of time it takes to cool the storage medium will also be about 4.2 ns, which is equal to the shortest write current pulse width. Consequently, marks could not be made properly depending on the property of the storage medium.
According to the second conventional technique mentioned above, each write code is broken down into a plurality of primitive elements with multiple different lengths such that a single write pulse is associated with each of those elements and that each write code is formed by a series of recording marks associated with respectively independent write pulses. However, this conventional technique does not consider thermal balance between write pulses for respective elements at all. That is why as the recording linear density is increased, it becomes more and more difficult to control the mark edge position. That is to say, in making marks that will form a single write code, the recording marks will have variable widths from one position to another because the quantity of heat accumulated in the recording layer for the top portion of the write code is different from that of heat accumulated there for the terminal portion of the write code. As a result, the edge recording cannot be carried out as intended.
In the third conventional technique, a pulse, which is much shorter than the detection window width, may be inserted into the write pulse waveform in the vicinity of the center of the mark making period, and the mark width changes significantly around there compared to the other portions. According to the document disclosing this conventional technique, when a mark edge recording operation is carried out, the variation in signal amplitude around the center portion of a mark should cause no serious problem as long as the mark edge position is accurate. In a read/write drive that determines read/write conditions by detecting the average level of a read signal, however, such distortion of the read signal should affect the operation of the drive. As to a phase change storage medium, for example, a signal can be detected as a variation in reflectance just like a phase pit type storage medium. That is why the phase change storage medium and phase pit type storage medium can easily share the same read drive in common. However, since the read signal of the phase pit type storage medium has no such distortion, it is actually difficult to read the phase change storage medium and phase pit type storage medium using the same drive.
Also, according to the fourth conventional technique, the write power level of the write pulse train changes stepwise, thus requiring complicated power control. Also, in writing a signal with a code length of 4 Tw, the laser beam needs to be emitted so as to achieve a higher power level than the average power level at least for a period of time corresponding to 3 Tw. When a very small mark needs to be made on a high-density storage medium in the near future, such an emission time will be too long to make desired recording marks.
As can be seen, none of the conventional techniques mentioned above can contribute to making marks sufficiently accurately when the transfer rate is high or achieving sufficiently high storage plane density and reliability.
In order to overcome the problems described above, an object of the present invention is to provide a method and apparatus for recording data that can make marks with high accuracy even when the transfer rate is high.